1. Field of the Invention
This invention relates to a rotating anode x-ray tube device.
2. Background of the Prior Art
Ordinary x-ray tubes are used for medical purposes, such as x-ray diagnosis, for example, but for examination of the stomach, etc., x-ray tubes such as the one shown in FIG. 7 are in use. This x-ray tube, a rotating anode x-ray tube, has a cathode 2 at one end of an envelope 1, with a cup 3, containing a cathode filament which emits thermal electrons and focusing electrodes set eccentrically. Towards the centre of the envelope 1, a disc-shaped anode target 4 is set facing the cathode 2. This anode target 4 is set at a large potential difference from the cathode 2 described above, causing the electrons emitted by the cathode filament to accelerate, collide, and produce x-rays by bremsstrahlung. In addition, in order to store and radiate the large amount of heat generated at this point, the anode is made to rotate at a high speed to effectively increase the area over which heat is generated. This sort of anode target 4 is continuous with a closed-end tube-shaped rotor 6, through a supporting rod 5. This rotor 6 is rotated by a rotating magnetic field produced by the stator 7 outside the envelope 1, and thus together they form an inductive motor. The supporting rod 5 and rotor 6 are a single unit. On the inside, rotor 6 has an axle 8 along its axis, and this axle is fixed to the rotor 6 by bolts, etc. (not shown). There is a closed-end tubular stator 9 between this axle 8 and the rotor 6, fixed to the envelope 1 through sealing rings 10, 11. Part of this stator 9 protrudes from the tube, and can be used as an external support and fixing point for the whole x-ray tube. Bearings 12, 13 are positioned between the stator 9 and the axle 8 so as to allow the axle 8 to rotate freely. In operation, when the electrons emitted from the cathode filament arrive at the target, the power reaches 1 kW for an anode voltage 50 kV and current 20 mA. Since more than 99% of this power is converted to heat, the anode is heated to a high temperature even with radiation of heat to the outside and conduction of heat to other components. Because thermal radiation increases in proportion to the 4th power of the temperature, at a high temperature the radiation greatly increases, soon reaching thermal equilibrium. For example, under the above conditions, an equilibrium is reached at 1100.degree. C. after 5 minutes. On the other hand, for heat transmission by conduction, with the other end of the conducting medium thermally free, the end gradually reaches a high temperature over a longer period. Thus, the heat from the target 4 is transmitted by the rotor 6 and axle 8, making them a high temperature. When the rotor 6 reaches a high temperature, thermal radiation increases and a thermal equilibrium is reached in the same way as above. Under the above conditions point B on the supporting rod 5 reaches thermal equilibrium at 800.degree. C. approximately 15 minutes after the power is switched on, point C on the rotor 6 at 550.degree. C. approximately 30 minutes after the power is switched on, and point D close to the bearing 12 at 400.degree. C. approximately 50 minutes after the power has been switched on. If the thermal conductivity of the bearing 12 is lower, the temperature at point D becomes the same as point C, reaching 550.degree. C. The balls in the bearings 12, 13 undergo thermal expansion with their rotation, causing deterioration of the clearances between them and the inner and outer wheels, causing possible problems. Also, if the bearings 12, 13 exceed 500.degree. C., this causes a reduction in the hardness of the balls, leading to tube breakdowns such as the rotation stopping.
With the temperature of the anode target 4 maintained at 800.degree. C.-1200.degree. C. during heat input, the amount of heat radiated from the anode target is different according to surface area, surface emissivity and shape factors, but is normally 2 kw-4 kW. However, if the temperature of the anode target 4 is reduced, since the radiated heat is greatly reduced in proportion to the 4th power of the absolute temperature, it takes a very long time to be sufficiently cooled.
On the other hand, a method for solving this problem, rotating anode x-ray tubes lowering the temperature of the anode target by letting a fluid coolant (eg, water) flow onto the anode target has already been made public in, for example, U.S. Pat. No. 2,926,269 (Broad) etc. These are constructed with the coolant flowing directly into the metal anode target, so that the anode target is maintained at the same earth potential as its housing.
However, existing x-ray tubes such as these have the following defects. As described above, the inner wheel of bearings 12, 13 easily reaches a high temperature, but the outer wheel is at a low temperature. At this point the temperature changes from 60.degree. C. to 550.degree. C. depending on the rotation of the balls in bearings 12, 13. When the balls are at a high temperature, not only does the clearance between the balls and the inner and outer rings become insufficient, but the lubricant between them can vaporize, causing damage to the bearings 12, 13. Because of this there is the defect that stopped rotation breakdowns occur easily and frequently. In order to prevent this, increasing the level of blackening of the target 4, increasing the level of blackening of the rotor 6 surface and providing a thermal shield between the target 4 and the rotor 6 have been suggested, but their effects are relatively small, and definitely make the input power to the target 4 too small.
In addition to this, the permissible temperature of the section of the anode target 4 which is struck by electrons emitted by the electron gun 3 and accelerated with a high voltage (electron incident surface) must be kept below 2800.degree. C. when the anode target 4 is made of tungsten, so as to prevent recrystallization. As above, since the temperature of the anode target as a whole rises to 800.degree.-1200.degree. C., the temperature of the ring-shaped section of the anode target 4 heated by the electrons (electron incident track surface) normally reaches 1200.degree.-1500.degree. C. Accordingly, the maximum value dT for the temperature rise of the electron incident surface due to the electrons striking is limited to 1300.degree.-1600.degree. C., and because the possible input electron beam power, and thus the x-ray output level are proportional to dT, they are restricted to a low value. This is particularly noticeable when the electron incident surface and thus the x-ray focus are small.
Since, as described above, the power of radiation from the anode target 4 reduces in proportion to the 4th power of the absolute temperature when the temperature of the target drops, the speed at which the temperature of the anode target 4 drops is extremely slow, and in order for the anode target 4 to reach a sufficiently low temperature, it must be left for a very long period.
In the case of the example in the above-mentioned Broad Patent (U.S. Pat. No. 2,926,269), because the anode target is at the same earth potential as the housing, for medical use the cathode potential would have to be from 0--150 kV, which not only means that a large and expensive high voltage power source is required, but that the cables are thick and cannot be used in an x-ray device using this x-ray tube.